Use a two channel scope. Measure both signals with respect to ground.
Each signal, or both by adding, them will show common mode noise.
Inverting one signal and adding them will show the differential noise.
Common mode is trace A -or- trace B -or- the result of adding them.
Differential mode is the sum of trace A minus trace B, in other words the difference between them.
Brian.
Hi,
if your power supply has only two terminals (like a battery) then it can´t generate commom mode noise.
Common mode noise also needs a reference to which you refer the voltage.
Form your description I assume this is an AC/DC or a DC/DC supply.
If so, then you can reference it to:
* EARTH_GND
* maybe even to the metal (shielding) case. Don´t know if this makes sense.
* also possible: one input voltage signal - mainly for DC/DC converters.
If AC/DC then take care about eventually dangerous voltages. Don´t create short circuit with the connections..
Maybe read through this: https://micro.rohm.com/en/techweb/knowledge/emc/s-emc/01-s-emc/6899
Klaus
Unlikely.The SMPS adapter I have has only two terminals
Hi,
Unlikely.
An SMPS usually has at least two input terminals and two output terminals. thus 4 in total.
Then there may be additionally: EARTH_GND, shielding, remote control, other voltages....
We do not know how your SMPS looks like until you show us. All the SMPS I/Os.
Klaus
Im talking about output DC terminals + and - Of course it has mains input mine doesnt have earth. It has two input AC terminals.
The CM is capacitevely coupling as I said.
Hi,
Yes - capacitively coupling - to the AC side.
This is the only way in your case for common mode noise on the DC side .. referenced to the AC side.
Thus if you want to measure CM noise you need to tke the AC side into account. Otherwise it can´t be CM noise.
Did you go through the link I sent in post#9? It clearly tells about CM noise.
Klaus
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